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Integrative square-grid triboelectric nanogenerator as a vibrational energy harvester and impulsive force sensor Chuan He 1,2 , Weijun Zhu 3,4 , Guang Qin Gu 1,2,5 , Tao Jiang 1,2 , Liang Xu 1,2 , Bao Dong Chen 1,2 , Chang Bao Han 1,2 , Dichen Li 3,4 , and Zhong Lin Wang 1,2,6 ( ) 1 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China 2 CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, China 3 State Key Laboratory of Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi’an 710049, China 4 Collaborative Innovation Center of High-End Manufacturing Equipment, Xi'an Jiaotong University, Xi’an 710049, China 5 University of Chinese Academy of Sciences, Beijing 100049, China 6 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA Received: 24 April 2017 Revised: 17 August 2017 Accepted: 25 August 2017 © Tsinghua University Press and Springer-Verlag GmbH Germany 2017 KEYWORDS square grid, triboelectric nanogenerator, vibration, sensor ABSTRACT A square-grid triboelectric nanogenerator (SG-TENG) is demonstrated for harvesting vibrational energy and sensing impulsive forces. Each square of the three-dimensional (3D)-printed square grid is filled with an aluminum (Al) ball. The grid structure allows the SG-TENG to harvest vibrational energy over a broad bandwidth and operate at different vibrational angles. The most striking feature of the SG-TENG is its ability of being scaled and integrated. After connecting two SG-TENGs in parallel, the open-circuit voltage and short-circuit current are significantly increased over the full vibrational frequency range. Being integrated with a table tennis racket, the SG-TENG can harvest the vibrational energy from hitting a ping pong ball using the racket, where a direct hit by the racket generates an average output voltage of 10.9 ± 0.6 V and an average output current of 0.09 ± 0.02 μA. Moreover, the SG-TENG integrated into a focus mitt can be used in various combat sports, such as boxing and taekwondo, to monitor the frequency and magnitude of the punches or kicks from boxers and other practitioners. The collected data allow athletes to monitor their status and improve their performance skills. This work demonstrates the enormous potential of the SG-TENG in energy harvesting and sensing applications. 1 Introduction Aiming at harnessing small-scale ambient energy, various energy harvesters such as thermoelectric generators (TEGs) [1, 2], piezoelectric nanogenerators (PENGs) [3–5], and triboelectric nanogenerators Nano Research https://doi.org/10.1007/s12274-017-1824-8 Address correspondence to [email protected]
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Page 1: Integrative square-grid triboelectric nanogenerator as a ... · PDF fileIntegrative square-grid triboelectric nanogenerator as a vibrational energy harvester and impulsive force sensor

Integrative square-grid triboelectric nanogenerator as avibrational energy harvester and impulsive force sensor

Chuan He1,2, Weijun Zhu3,4, Guang Qin Gu1,2,5, Tao Jiang1,2, Liang Xu1,2, Bao Dong Chen1,2, Chang Bao Han1,2,

Dichen Li3,4, and Zhong Lin Wang1,2,6 ()

1 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China 2 CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, China 3 State Key Laboratory of Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi’an 710049, China 4 Collaborative Innovation Center of High-End Manufacturing Equipment, Xi'an Jiaotong University, Xi’an 710049, China 5 University of Chinese Academy of Sciences, Beijing 100049, China 6 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA

Received: 24 April 2017

Revised: 17 August 2017

Accepted: 25 August 2017

© Tsinghua University Press

and Springer-Verlag GmbH

Germany 2017

KEYWORDS

square grid,

triboelectric nanogenerator,

vibration,

sensor

ABSTRACT

A square-grid triboelectric nanogenerator (SG-TENG) is demonstrated for

harvesting vibrational energy and sensing impulsive forces. Each square of the

three-dimensional (3D)-printed square grid is filled with an aluminum (Al) ball.

The grid structure allows the SG-TENG to harvest vibrational energy over a

broad bandwidth and operate at different vibrational angles. The most striking

feature of the SG-TENG is its ability of being scaled and integrated. After

connecting two SG-TENGs in parallel, the open-circuit voltage and short-circuit

current are significantly increased over the full vibrational frequency range.

Being integrated with a table tennis racket, the SG-TENG can harvest the

vibrational energy from hitting a ping pong ball using the racket, where a direct

hit by the racket generates an average output voltage of 10.9 ± 0.6 V and an

average output current of 0.09 ± 0.02 μA. Moreover, the SG-TENG integrated

into a focus mitt can be used in various combat sports, such as boxing and

taekwondo, to monitor the frequency and magnitude of the punches or kicks

from boxers and other practitioners. The collected data allow athletes to monitor

their status and improve their performance skills. This work demonstrates the

enormous potential of the SG-TENG in energy harvesting and sensing

applications.

1 Introduction

Aiming at harnessing small-scale ambient energy,

various energy harvesters such as thermoelectric

generators (TEGs) [1, 2], piezoelectric nanogenerators

(PENGs) [3–5], and triboelectric nanogenerators

Nano Research

https://doi.org/10.1007/s12274-017-1824-8

Address correspondence to [email protected]

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2 Nano Res.

(TENGs) [6–14], have been developed as power sources

for wearable electronics and sensor networks. Among

them, TENG is of great interest for capturing low-

frequency mechanical energy owing to its low-cost

fabrication and excellent robustness [15, 16]. Based

on the coupling of the triboelectric effect and elec-

trostatic induction, different TENG structures have

been designed for harvesting mechanical energy of

different forms, e.g., human motion [9, 10, 12], vibration

[6–8], and water waves [11]. Among these, vibration

is one of the most common mechanical motions

ubiquitously available in our living environment.

Spring-assisted TENGs were previously introduced

to harvest vibrational energy, where the springs were

used to move the triboelectric layers apart when the

TENGs were subjected to external vibrations [6–8].

However, there are certain limitations to spring-

assisted structures. First, the use of springs typically

results in a relatively bulky volume of the TENGs.

Secondly, an additional space is required for the

vibrational part. Thirdly, considerable vibrational energy

loss might occur when the springs become loose.

To avoid these limitations, Wen et al. [11] reported

a wavy TENG structure that allows self-restoration

without using extra springs. In addition, package

structures of TENGs based on the internal vibration of

the oscillators have also been reported. The oscillators

that have been used thus far are polymer-coated metal

[17], polytetrafluoroethylene (PTFE) powders [18], or

ferrofluid [19].

In this article, we demonstrate a square-grid TENG

(SG-TENG) for harvesting vibrational energy and

sensing impulsive forces. The demonstrated SG-TENG

employs a package structure and uses a 3D-printed

square grid as the frame. Each square of the grid is

filled with an aluminum (Al) ball as the oscillator.

The grid structure allows the SG-TENG to operate at

different vibrational angles. Owing to the special design

and small size, the SG-TENG can easily be scaled

and integrated into other structures. By connecting

two SG-TENGs in parallel, both open-circuit voltage

(VOC) and short-circuit current (ISC) are greatly increased

over the entire vibrational frequency range. Furthermore,

when integrated with a table tennis racket, the

SG-TENG can harvest the vibrational energy of the

racket from hitting ping pong balls during game play.

Moreover, the SG-TENG integrated into a focus mitt

can not only count the total number of punches

but also track the force applied in every impact. The

acquired data allow athletes to monitor their status

and improve their performance during training.

2 Results and discussion

The structural design of the SG-TENG is schematically

illustrated in Figs. 1(a) and 1(b). As shown in Fig. 1(a),

the SG-TENG has a sandwich structure consisting

of four parts. In the middle, a 30 × 30 square grid

frame (83 mm × 83 mm × 2 mm) is fabricated using

stereolithography (SL) and filled with Al balls. The Al

balls act as both an oscillator and electropositive

triboelectric layer. On each side of the square grid is a

layer of PTFE film placed on top of an Al plate. Here,

the PTFE film was chosen as the electronegative layer

owing to its ability to attract electrons, whereas the

Al plate acts as both an electrode and a protective

layer. It can be seen in Fig. 1(b) that each square of

the grid contains a single Al ball and the side length

of the square and the diameter of the Al ball are 2

and 1 mm, respectively. This structure allows the

vibration of the Al balls inside the SG-TENG when

an external vibration is applied. Figures 1(c) and 1(d)

show photographs of the side view of the SG-TENG

and the front view of the square grid, respectively.

The detailed fabrication process of the SG-TENG is

presented in the Experimental section.

Figure 2 illustrates the working principle of the

SG-TENG. Considering that the SG-TENG has a grid

structure, we only demonstrate one square unit for

Figure 1 Structural design of the SG-TENG. (a) Schematic illustration of the device structure. (b) The square grid and Al balls inside. (c) Photograph of the side view of the SG-TENG. (d) Photograph of the front view of the 3D-printed square grid.

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3 Nano Res.

Figure 2 The working principle of the SG-TENG.

clarification. According to the triboelectric series,

when the Al ball comes into contact with the PTFE

films, the electrons will inject from the Al ball to the

surfaces of the PTFE films through triboelectrification

[11]. Thus, the total amount of positive triboelectric

charges on the Al ball should be the same as the

negative triboelectric charges on the PTFE films.

Initially, as shown in Fig. 2(i), the Al ball is in contact

with the PTFE film at the bottom, and thus the

negative charges are attracted to the bottom electrode,

leaving the same amount of positive charge on the

top Al electrode. The Al ball then moves upward

when the SG-TENG is subjected to an external

vibration. As the Al ball approaches the top, the

top electrode has a higher potential than the bottom

electrode, and hence the electrons transfer from the

bottom to the top in the external circuit (see Fig. 2(ii)).

Once the Al ball reaches the top PTFE film, all electrons

are transferred to the top electrode, as shown in

Fig. 2(iii). When the Al ball takes the reverse course, a

reverse transfer of the electrons occurs through

the external circuit (see Fig. 2(iv)). Finally, the Al ball

returns to the bottom of the PTEF film, and a full

cycle is completed (see Fig. 2(i)).

To evaluate the performance of the SG-TENG, an

electrodynamic shaker (Labworks, Inc.) was used to

produce sinusoidal vibrations with a fixed amplitude

and tunable frequency. After attaching the SG-TENG

to the shaker, the output dependence of the SG-TENG

on the vibration frequency was measured at vibrational

angles of 0°, 45°, and 90°. The vibrational angle, α,

is defined as the angle between the surface of the

SG-TENG and normal to the ground. It should be

noted that the surface of the SG-TENG is always

perpendicular to the vibrational direction. As previously

described, the sinusoidal vibration applied to the

SG-TENG induces the vibration of the Al balls inside,

and hence an alternating current (AC) is produced in

the external circuit. Figures 3(a)–3(c) show the electrical

output of the SG-TENG at α = 0°, 45°, and 90°,

respectively. The VOC and ISC of the SG-TENG over a

vibration frequency at α = 0°, 45°, and 90° are plotted

in Figs. 3(a)(i), 3(b)(i), and 3(c)(i), respectively. The

vibrational frequency ranges from 10 to 180 Hz,

which covers most of the ambient vibrations in our

daily life [20]. At α = 0°, 45°, and 90°, the bandwidths

for the voltage are 92.6, 89.3, and 88.3 Hz, respectively,

and for the current are 61.94, 84.6, and 103.3 Hz,

respectively; in addition, the measured peak-to-peak

values of VOC are 3.76, 5.66, 5.28 V, respectively, whereas

the amplitudes of ISC are 0.37, 0.41, and 0.39 μA,

respectively. We can see that as α increases from 0°

to 90°, the electrical output also increases over the

vibrational frequency, particularly within the low

frequency range, which leads to an increase in the

bandwidth. The reason for this can be ascribed to the

fact that, at an angle of 90°, the Al balls are at rest at

the bottom of the SG-TENG, and thus more vibrational

energy can be transferred to them than to the Al balls

at an angle of 0°, which are at rest at the side of the

SG-TENG.

Moreover, VOC and ISC of the SG-TENG in the time

domain at α = 0°, 45°, and 90° are also depicted in

Figs. 3(a)(ii) and 3(a)(iii), 3(b)(ii) and 3(b)(iii), and 3(c)(ii)

and 3(c)(iii), respectively. The electrical signals at 20,

40, 60, and 80 Hz are chosen here for comparison.

The insets in Figs. 3(a)(iii), 3(b)(iii), and 3(c)(iii) depict

the corresponding transferred charges between two

electrodes. As the frequency increases, the signal

evolves from a pulsed output to an oscillatory output;

in addition, the peak current also increases, whereas

the total charge transferred during one cycle remains

almost constant. At a frequency of 80 Hz, the total

charge transferred at α = 0°, 45°, and 90° is 1.65, 1.68,

and 1.35 nC, respectively. These results indicate that

the SG-TENG is capable of harvesting vibrational

energy over a broad bandwidth and at different

vibrational angles.

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4 Nano Res.

One of the features of the SG-TENG is its scalability,

and by connecting the SG-TENGs in parallel, the total

output can be increased with an increased number of

SG-TENGs. To prove this, we measured the frequency

response of two parallel-connected SG-TENGs. A

comparison of VOC and ISC for a single SG-TENG and

two parallel-connected SG-TENGs is illustrated in

Figs. 4(a)(i) and 4(a)(ii), respectively. The measurements

were performed at a vibrational angle of 90°. Clearly,

VOC and ISC of the two parallel-connected SG-TENGs

are greatly increased over the full vibrational frequency

range. Compared to the single SG-TENG, the parallel-

connection increases the contact area between the

triboelectric layers, which produces more triboelectric

charges, thus leading to an enhancement in the

electrical output. At a frequency of 80 Hz, the peak-

to-peak value of VOC and the amplitude of ISC are 2.4

and 3.9 times greater than those of a single SG-TENG,

respectively. Therefore, a higher electrical output is

expected as the number of SG-TENGs increases.

As mentioned before, owing to its small size, the

SG-TENG can be easily integrated into any vibrational

surfaces or structures for harvesting the vibrational

energy. For example, as shown in Fig. 4(b), we

demonstrated the ability of the SG-TENG for harvesting

the vibrational energy of a table tennis racket. Because

the SG-TENG is only 4 mm thick (see Fig. 1(c)), it

can be integrated into the racket; in this study, we

directly attached a SG-TENG to a racket for simplicity.

Figure 4(b)(i) shows an image of a ping pong ball

bouncing on the racket, with an image of the SG-TENG

being attached to the racket shown in the inset of

Fig. 4(b). The amounts of VOC and ISC generated by

the SG-TENG are shown in Figs. 4(b)(ii) and 4(b)(iii),

respectively, and enlarged views of the highlighted

VOC and ISC are shown in the corresponding insets. As

the ball bounces against the racket, a series of electrical

signals is generated each time the ball hits it (Video S1

in the Electronic Supplementary Material (ESM)). The

average output voltage of the SG-TENG is about 4.6 ±

Figure 3 The VOC and ISC of the SG-TENG at different vibrational angles α. (a) α = 0°, (b) α = 45°, and (c) α = 90°. The insets in (a)(iii), (b)(iii), and (c)(iii) show the corresponding transferred charges between two electrodes.

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5 Nano Res.

0.9 V, whereas the average output current is 0.19 ±

0.02 μA. Furthermore, the electrical performance of

the integrated SG-TENG during game play was also

evaluated by hitting a series of ping pong balls using

the racket (see Fig. 4(b)(iv)). The values of VOC and ISC

of the SG-TENG during consecutive strokes are shown

in Figs. 4(b)(v) and 4(b)(vi), respectively, where the

signals generated in a single stroke are highlighted. A

single stroke consists of two sequential processes: the

swinging of the racket and a direct hit on the ball by

the racket. As indicated in Figs. 4(b)(v) and 4(b)(vi),

the entire process is delicately disclosed in the obtained

signal. Because the swing process precedes the hitting

process in a single stroke, the electrical signal produced

is composed of two parts: signals generated by the

swing process and signals generated by the hitting

process that follows. During game play, the swing of

the SG-TENG produces an average output voltage

of 9 ± 1 V and an average output current of 0.05 ±

0.02 μA, whereas a direct hit by the racket generates

an average output voltage of 10.9 ± 0.6 V and an

average output current of 0.09 ± 0.02 μA (Video S2 in

the ESM). This demonstration proves that an integrated

SG-TENG can effectively harvest the vibrational energy.

In addition, the SG-TENG is sensitive to different

types of vibrations, i.e., swinging and hitting during

play, and thus the SG-TENG also has a great potential

in sensor applications.

In addition to harvesting vibrational energy, the

capability of the SG-TENG as an impulsive force sensor

is demonstrated. Figure 5(a)(i) shows the dependence

of ISC on the impulsive force applied on the SG-TENG.

It is clear that the amplitude of ISC increases as the

impulsive force increases. The relationship between

Figure 4 (a) Comparison of (i) the VOC and (ii) ISC for a single SG-TENG and two parallel-connected SG-TENGs. (b) Demonstration of the SG-TENG as a vibrational energy harvester. (i) Photograph of a table tennis racket with an integrated SG-TENG. As the ball bounces on the racket, VOC and ISC generated by the SG-TENG are as shown in (ii) and (iii), respectively. The insets are the respectively enlarged views of one cycle of VOC and ISC. (iv) The electrical performance of the integrated SG-TENG during game play evaluated when hitting a series of ping pong balls using the racket. The corresponding VOC and ISC of the SG-TENG are shown in (v) and (vi), respectively.

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6 Nano Res.

the amplitude of ISC and the impulsive force is plotted

in Fig. 5(a)(ii). From Fig. 5(a)(ii), we can see that the

amplitude of ISC is approximately linearly proportional

to the impulsive force. This linearity is crucial for the

SG-TENG as an impulsive force sensor. As is well

known, the impulsive force is commonly encountered

in various combat sports, such as boxing, taekwondo,

and kickboxing. Taking boxing as an example, a focus

mitt is a padded target that is generally used for

training boxers and other combat athletes. For this

study, we integrated a SG-TENG into the focus mitt,

as shown in Fig. 5(b). When the focus mitt is punched

repeatedly, a series of pulse signals is generated

(Video S3 in the ESM). The electrical signals of the

integrated SG-TENG punched by persons A and B

are illustrated, where VOC and ISC generated by person

A are shown in Figs. 5(b)(i) and 5(b)(ii), respectively,

and VOC and ISC generated by person B are shown in

Figs. 5(b)(iii) and 5(b)(iv). The insets in Figs. 5(b)(i)

through 5(b)(iv) show enlarged views of the corres-

ponding highlighted pulse signals. Because each

punch produces a pulse signal, the SG-TENG can be

utilized to count the total number of punches during

training. In the meantime, as indicated in Fig. 5(a),

the SG-TENG is also able to track the force of every

punch. The frequency and magnitude of the punches

collected by the SG-TENG can help athletes monitor

their status and improve their performance during

training.

3 Conclusions

In conclusion, we demonstrated a square-grid TENG

as a vibrational energy harvester and an impulsive

force sensor. The design of the SG-TENG allows it to

harvest the vibrational energy over a broad bandwidth

at different vibrational angles (α = 0°, 45°, and 90°).

The SG-TENG can be easily scaled, and by connecting

Figure 5 Demonstration of the SG-TENG as an impulsive force sensor. (a) (i) The ISC of the SG-TENG under different impulsive forces. (ii) Amplitude of ISC as a function of impulsive force and the linear fit of the experiment data. (b) Photograph of a focus mitt withan integrated SG-TENG as an impulsive force sensor. The VOC and ISC of the focus mitt punched repeatedly by persons A and B.

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7 Nano Res.

two SG-TENGs in parallel, the values of both VOC

and ISC are greatly increased over the full vibrational

frequency range. At a frequency of 80 Hz, the peak-

to-peak value of VOC and the amplitude of ISC are 2.4

and 3.9 times as large as those of a single SG-TENG.

By integrating the SG-TENG into a table tennis racket,

the SG-TENG can harvest vibrational energy from

hitting the ping pong balls using the racket. A direct

hit by the racket generates an average output voltage

of 10.9 ± 0.6 V and an average output current of 0.09 ±

0.02 μA. Furthermore, the SG-TENG can be used as

the impulsive force sensor. We demonstrated the

ability of applying the SG-TENG to boxing training

to count the total number of punches and track the

force of every punch. Owing to its lightness in weight

and thinness, as well as its capability of being scaled

and integrated, the SG-TENG has significant potential

in energy harvesting and sensing applications.

4 Experimental section

The SG-TENG consists of a square grid frame, Al

balls, and two polytetrafluoroethylene (PTFE) films

on Al plates. The 30 × 30 square grid has a thickness

of 2 mm and a side length of 83 mm. The square grid

is fabricated using epoxy acrylate with a SL, which

has a resolution of ±0.1 mm. The side length of the

square is 2 mm. For each square, there is an Al ball

with a diameter of 1 mm filled inside. On each side of

the square grid is a layer of PTFE film placed on top

of the Al plate. The Al plate has a thickness of 1 mm,

whereas the thickness of the PTFE film is 80 μm. All

of the electrical measurements of the SG-TENG were

applied using a Keithley 6514 System Electrometer. A

function generator (Stanford Research Systems DS345)

and a linear power amplifier (Labworks PA-141) were

used to produce the sinusoidal oscillations. A vibration

shaker (Labworks ET-126B-4) was used to simulate

mechanical vibration. The dynamic force applied was

measured using a force gauge (HP-50) mounted on a

linear motor.

Acknowledgements

Supports from the “thousands talents” program for

the pioneer researcher and his innovation team, the

National Key R&D Project from Minister of Science

and Technology, China (No. 2016YFA0202704), National

Natural Science Foundation of China (Nos. 51432005,

51608039, 5151101243, 51561145021, and 51505457),

China Postdoctoral Science Foundation (No.

2015M581041), and Natural Science Foundation of

Beijing, China (No. 4154090) are appreciated.

Electronic Supplementary Material: Supplementary

material (Videos S1–S3 demonstrate the electrical

performance of the SG-TENG that being integrated

into the table tennis racket and the focus mitt) is

available in the online version of this article at

https://doi.org/10.1007/s12274-017-1824-8.

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